Process for treating methyl acetate



Aug. 25,1953 M. MENTION ETAL PROCESS FOR TREATING METHYL ACETATE Filed April 1o.n 1951 Patented Aug. 25, 1953 Maurice Mention France, assignors` and Jules Mercier,

to Les Usines Melle, de Melle ('Societe Anonyme), Saint -Lcger-'les-Melle; France; i a` corporation ofFrance Application April'l, 1951, Serial N0. 220,291

In France Apr 4 Claims. (Cl. 260541) This invention relates to theseparation of mixtures of ester; Water, and acidand alcohol, capable of forming such ester. More particularly it relates to the separation into their components of mixtures containing methyl acetate, methyl alcohol, acetic acid` and aqueous mineral acid.

One of the sources for the mixture of components just mentioned results from the hydrolysisof methyl acetate which is a ley-product in the manufacture of polyvinylalcohol. Heretofore this methyl acetate so obtained has been saponiiied by means of sodiumA hydroxide with a later addition of a strong acid such as sulphuric acid in calculated amount toA liberate the acetic` acid, whereupon the methyl alcohol and acetic acid are separatedl by distillation. This method is somewhat complicated and expensive,

It is an object of this invention, accordingly, to provide a simple, efficient process for treating methyl acetate and recovering methyl alcohol and acetic acid.

The invention accordingly comprises the novel processes and steps of processes,` specific embodia ments of which are described hereinafter by way of example and in accordance with which we now prefer to practice the invention. i

We have found in accordance With out invention that it is possible to subject the methyl acetate to hydrolysisin the presence of a catalyst under superatmospheric pressure at a temperature approximating but lower than the boiling point. In order` to carry out this hydrolysis We subject aqueous methyl acetate to which has been added a small proportion, for example about 1% by weight (or a somewhat higher or lower proportion) of a catalyst such as sulphuric acid, to heat under superatmospheric pressure bringing the mixture of methyl acetate, sulphuric acid and` water to a temperature approximating the boiling point but below such boiling point at the pressure selected. For example We have found it suitable to work at about 100 C. under a pressure of 3. kgs/cm.2 above atmospheric pressure. Or We may operate at about 115 C. under a pressure of 6 kgs/cm.2 above atmospheric pressure. This process can` With advantage be carried out continuously; in that case, methyl acetate in admixture with water and catalyst is passed through a zone in which prevail the pressure and temperature as defined above, the proportion of water with respect to ester being such that the ester is only partly hydrolyzed; in practice, We use 0.3.1:'2 parts of Waterfor one part of ester (by Weight), the most favorable proportions being l to 1.2

parts of Water for one part of ester. The timef of about 10 toy 45' of residence in this zone isA minutes.

The reason why the ester should be only part- 1y hydrolyzed isthat total hydrolysis of the ester would require too long time. @n the other' hand the estershould nevertheless` byA hydrolyzedl up to an extent suilicient for the operation toM be commercially benecial" In practice; we pre-- fer that only\20% toV 70% by weight off the ester be hydrolyzed, more particularly ab out In accordance with out invention We conduct a process of distillingand` refluxing a mixture ofv methyl acetate, methyl alcohol; acetic` acidl andi aqueousmineral acid toseparate an aqueous mixture of methyl acetate` and methyl alcohol from the other ingredients. Theother ingredients include the acetic acidand the aqueous mineral acid. We4 thenproceedL with di'stillihg` and scrub-- bing with water the vapors of the aqueous mixi-` ture of methyl acetate and methyl alcohol1 tol separate the twolast named compounds from one another. Wethen extract acetic acid to=separate it from the minerali acid and distill the extract. As stated the mixtureofm'etliylf acetate, methylV alcohol, aceticaoid` and Water containing the mineral acid such as sulphurio acid maybe obtained from partial hydrolysis of an aqueous solution of methyl acetate, and more specifically the process of recovering acetici acid and methyl alcohol is carried out in the following manner: (a) distilling` themixturewhile condensing part of the vaporsand-reluxing the condensate into= the distilling column, thus obtaining atl the top of the column` methyl acetate and methyll alco` hol vapors and, atthe foot thereof, aqueousacetic acidi; (b) scrubbing the` last-named vapors with water, which carries olfthe methyl alcohol in the formA of an aqueous solution thereof, thus separating the methyl alcohol from the methyl acetate, and `(c)4 concentrating and dehydrating the aqueous acetic acid through extraction. by means of a solvent `which acts subsequentlyas a water-entrainer; then distilling the extracto to recover aceticacid. In accordance withl a fea-` ture of the invention, we use for carrying out step (c), at least a part of thenon-hydrolyzed methyl acetate as the solvent and water-en.- trainer, preferably methyl acetate` which: 'has beenl separated instep `(b). Instead of using methyl acetateseparated ,inf step (b1)` wemay use some of the raw` methylF acetate priorito li-ydrolysis. i n

In the drawingforming partof this application there is" shown a diagrammatic view of appara@ A tially free from volatile products.

. to these columns.

tus in which the process of our invention may be carried out. A specific example of the process and an apparatus which may be used in carrying it out is described below. It to be understood that such example is illustrative and that the invention is not to be limited thereto except as indicated in the appended claims.

Referring now to the drawing: R is a reaction vessel or reactor, which is fed with methyl acetate from tank A and with a 2% aqueous solution of sulphuric acid from tank B, the methyl acetate and sulphuric acid being delivered in the proportion of about 125V parts byV weight of sulphuric acid of the concentration mentioned to 100 parts by weight of methyl acetate. The temperature in reactor R is 100 C. and the pressure, 3 kgs/cm?. The liquid mixture of methyl acetate passes upward through pipe 28 which has a release valve therein for releasing the pressure prevailing in reactor R, and is sent into distilling column I. From the foot of column I there flows a mixture of water, acetic acid and mineral acid, namely sulphuric acid, through the pipe I9, this mixture o acetic acid and mineral acid being substan- At the top ofV column I vapors are evolved, part of which are condensed in condenser 8 and refluxed through pipe 9. The refluxed condensate prevents vapors of acetic acid from reaching the top of the column I and passing off with the methyl alcohol and methyl acetate. The remainder of the vapors pass through pipe I into the middle of distilling column 2 for the separation of methyl acetate and methyl alcohol.

Methyl acetate and methylalcohol form an azeotropic mixture, but this is avoided and the ester is separated'from the alcohol by supply-V ing water to the head of column 2. This water,

for example, is warm water flowing from the foot of column 3 vmentioned below and introduced into the top of column 2 through pipe II. This 'Water causes a separation ofthe ester from the alcohol by hydroselectiom The ester concentrates at the top of the column 2 in the form of ester-water azeotrope. The vapors of methyl acetate leaving the top of column 2 are condensed in condenser I2, a portion of the condensate being reiiuxed through pipe I3 to the top of the columnv 2 whileV the remainder is returned to the reaction vessel R through pipe I4 together with fresh crude methyl acetate from tank A.

At the foot of column ,2 there is obtained an aqueous solution of methyl alcohol which is introduced through pipe I5 into the middle part of distilling column 3. Distillingcolumns I', 2 and 3 are each provided respectively with steam coils Ia, 2a and 3a for the purpose of providing heat Methyl alcohol leaves column 3 at the top thereof throughjpipe 3b and is condensed in condenser I6, a portionvo the condens'ate being refluxedV through pipe II while the remainder is collected at I8.

The aqueous acetic acid obtained'at the foot of column VI along with the sulphuric acid is introduced through pipe I9 into extracting column 4 in which it flows downward countercurrently with respect to a solvent consisting of a mixture of methyl acetaterand cyclohexane containing V10-25% by weight of cyclohexane. lCyclohexane is introduced into the apparatus at the beginning of the operation Vthrough valved pipe 2l. The waterfreed from acetic acid but containing the Y sulphuric acid and traces of methyl acetate is returned through pipe 20 from the bottom of exf tracting column 4 to the inlet of the reactorR.

n decantingdevice B. The aqueous lower layer is returned from the decanting device to the inlet Y of .reaction vessel R through pipe 23 while the upper layer of solvent is returned through pipe 24 to extracting column 4 in which it is used again for the extraction of further acetic acid.

It is not necessary to further treat the residual water that Vflows out ofcolumn 4 through Vpipe 20 when methyl acetate is used as the solvent. In addition if methyl acetate is partly hydrolyzed in the extracting column under the action of the sulphuric acid there present, this hydrolysis is not detrimental. On the other hand if another ester than methyl acetate were employed as the solvent in this step in column Il the apparatus would thereupon become contaminated with'an alcohol other than methyl alcohol and it would be necessary to re-esterify it in another apparatus.

In addition it is pointed out that cyclohexane is Avery slightly soluble in water saturated with methyl acetate so that small quantitiesY of cyclohexane flow into reaction vessel R. This does not interfere with the reaction. However it is Vobvious that after leaving the reaction vessel the cyclohexane will go successively to the top of column I, then to the top of column 2. Pipe' is providedlto recover it in part. This pipe returns such cyclohexane and methyl acetate to the lower part of column 4. It is pointed out that it is necessary to compensate by this means for the methyl acetate loss which occurs by the return of the methyl acetate through pipe 20 rto the reactor R. Y

The adding of a substance such as cyclohexane to a solvent such as methyl acetate in connection with the recovery of acetic acid from aqueous solutions thereof is not claimed herein as it is not the invention of the present applicants. Such a process is disclosed and claimed in the cepending application of Maurice VMention and Louis Y@presence of a strong mineral acid, at a tempera'- ture approximating, butbelow, the boiling pointl of themixture at the pressure employed,V to produce amixture Vof methyl acetate, aceticl acid,

methyl alcohol and aqueous mineral acid, subjecting the last-named mixture as such to distillation and refluxing, to separate an aqueous mixture of methyl acetate and Ymethyl alcohol Vfrom the other ingredients, subjecting said aqueous mixture to extractive distillation in aY distilling column with water fed to the top section of the column to scrubrthe vapors andV separate Vmethyl alcohol from methyl acetate, distilling methyl alcohol to separate'it fromwater, extracting the aqueous acetic acid with a Ysolvent serving as a water-entrainer and comprising methyl acetate,V distilling the acetic'acidso extracted to concentrate and dehydrate same, and

i 5 recycling the aqueous mineral acid to the hydrolyzing step.

2. A process in accordance with claim 1, in which the methyl acetate in the solvent serving as a Water-entrainer is taken from the methyl acetate separated from the methyl alcohol after scrubbing of the vapors of the aqueous mixture thereof.

3. A process in accordance with claim 1, in which the mixture is lpassed continuously through a zone in which the pressure and temperature mentioned exist, the time of residence of the mixture in said zone being about 10 to about 45 minutes and the proportion of water with respect to ester being such that about 20% to 70% of the ester is hydrolyzed.

4. A process in accordance with claim 3, in which the proportion `of Water with respect to hydrolyzed.

MAURICE MENTION. JULES MERCIER.

References Cited in the 111e of this patent Number Number UNITED STATES PATENTS Name Date Retze Nov. l, 1927 Krchma June 13, 1933 Allquist July 11, 1939 Bright July 21, 1942 Bartlett 1 Dec. 18, 1945 FOREIGN PATENTS Country Date France Aug. 21, 1929 France Feb. 14, 1944 

1. A PROCESS WHICH COMPRISES PARTIALLY HYDROLYZING UNDER PRESSURE METHYL ACETATE IN THE PRESENCE OF A STRONG MINERAL ACID, AT A TEMPERATURE APPROXIMATING, BUT BELOW, THE BOILING POINT OF THE MIXTURE AT THE PRESSURE EMPLOYED, TO PRODUCE A MIXTURE OF METHYL ACETATE, ACETIC ACID, METHYL ALCOHOL AND AQUEOUS MINERAL ACID, SUBJECTING THE LAST-NAMED MIXTURE AS SUCH TO DISTILLATION AND REFLUXING, TO SEPARATE AN AQUEOUS MIXTURE OF METHYL ACETATE AND METHYL ALCOHOL FROM THE OTHER INGREDIENTS, SUBJECTING SAID AQUEOUS MIXTURE TO EXTRACTIVE DISTILLATION IN A DISTILLING COLUMN WITH WATER FED TO THE TOP SECTION OF THE COLUMN TO SCRUB THE VAPORS AND SEPARATE METHYL ALCOHOL FROM METHYL ACETATE, DISTILLING METHYL ALCOHOL TO SEPARATE IT FROM WATER, EXTRACTING THE AQUEOUS ACETIC ACID WITH A SOLVENT SERVING AS A WATER-ENTRAINER AND COMPRISING METHYL ACETATE, DISTILLING THE ACETIC ACID SO EXTRACTED TO CONCENTRATE AND DEHYDRATE SAME, AND RECYCLING THE AQUEOUS MINERAL ACID TO THE HYDROLYZING STEP. 